The present invention relates to an apparatus and method for projection exposure used for forming fine patterns in semiconductor integrated circuits, liquid crystal displays, etc.
A projection optical system used in a projection exposure apparatus of the type described above is incorporated in the apparatus after high-level optical designing, careful selection of a glassy material, ultraprecise processing, and precise assembly adjustment. The present semiconductor manufacturing process mainly uses a stepper in which a reticle (mask) is irradiated with the i-line (wavelength: 365 nm) of a mercury-vapor lamp as illuminating light, and light passing through a circuit pattern on the reticle is focused on a photosensitive substrate (e.g., a wafer) through a projection optical system, thereby forming an image of the circuit pattern on the substrate. Recently, an excimer stepper that employs an excimer laser (KrF laser of wavelength 248 nm) as an illuminating light source has also been used. When a projection optical system for such an excimer stepper only comprises a refracting lens, usable glassy materials are limited to quartz, fluorite, and so forth.
Generally speaking, in order to faithfully transfer a fine reticle pattern onto a photosensitive substrate by exposure using a projection optical system, the resolution and focal depth or depth of focus (DOF) of the projection optical system are important factors. Among projection optical systems which are presently put to practical use, there is a projection optical system having a numerical aperture (NA) of about 0.6 and employing the i-line. In general, when the wavelength of illuminating light employed is kept constant, as the numerical aperture of the projection optical system is increased, the resolution improves correspondingly. In general, however, the focal depth (DOF) decreases as the numerical aperture NA increases. The focal depth is defined by DOF=.+-..lambda./NA.sup.2, where .lambda. is the wavelength of illuminating light. It should be noted that when the illuminating light is reduced in wavelength, the resolution improves, but that the focal depth decreases with the reduction in wavelength.
In the meantime, even if the resolution is improved by increasing the numerical aperture NA of the projection optical system, the focal depth (focus margin) DOF decreases in inverse proportion to the square of the numerical aperture, as shown in the above expression of DOF=.+-..lambda./NA.sup.2. Accordingly, even if a projection optical system having a high numerical aperture can be produced, the required focal depth cannot be obtained; this is a great problem in practical use. Assuming that the wavelength of illuminating light is 365 nm of the i-line and the numerical aperture is 0.6, the focal depth DOF becomes a relatively small value, i.e., about 1 .mu.m (.+-.0.5 .mu.m) in width. Accordingly, resolution decreases in a portion where the surface unevenness or the curvature is greater than DOF within one shot region (which is about 20 by 20 mm or 30 by 30 mm square) on the wafer. In addition, it becomes necessary in the stepper system to perform focusing, leveling, etc. Fog each shot region on the wafer with particularly high accuracy, thereby resulting in an increase of the load (i.e., effort required to improve measurement resolving power, servo control accuracy, setting time, etc.) on the mechanical system, the electrical system and the software.
Under these circumstances, the present applicant has proposed in, for example, Japanese Patent Unexamined Publication (KOKAI) Nos. 04-101148 and 04-225358, a novel projection exposure technique whereby the above-described problems of the projection optical system are solved, and both high resolution and a large focal depth can be obtained without using a reticle provided with a phase shifter such as that disclosed in Japanese Patent Examined Publication No. 62-50811. This proposed exposure technique enables the apparent resolution and focal depth to increase by controlling the reticle illuminating method in a special mode without modifying the existing projection optical system. This technique is called SHRINC (Super High Resolution by IllumiNation Control) method. According to the SHRINC method, the reticle is irradiated with two illuminating light beams (or four illuminating light beams) which are symmetrically inclined in the pitch direction of a line-and-space pattern (L&S pattern) on the reticle, and the 0th--order diffracted light component and either of the .+-.1st-order diffracted light components, which are produced from the L&S pattern, are forced to pass through the pupil of the projection optical system in symmetry with respect to the optical axis, thereby producing a projected image (interference fringes) of the L&S pattern by utilizing the principle of two-beam interference (i.e., the interference between one of the 1st--order diffracted light components and the 0th--order diffracted light component). The image formation that utilizes two-beam interference makes it possible to suppress the occurrence of wavefront aberration when defocus occurs, in comparison to the conventional method (i.e., the ordinary vertical direction illumination). Therefore, the focal depth apparently increases.
However, the SHRINC method utilizes the interference of light between patterns which are relatively close to each other on the reticle, thereby improving the resolution and the focal depth. That is, the desired effect can be obtained when the pattern formed on the reticle has a periodic structure as in the case of an L&S pattern (grating pattern). However, no effect can be obtained for isolated patterns (in which the distance between patterns is relatively long), for example, contact hole patterns (fine square patterns). The reason for this is as follows: In the case of isolated fine patterns, diffracted light is produced therefrom in such a manner as to form a distribution which is approximately uniform in the direction of the angle of diffraction, and hence it does not clearly separate into the 0th--order diffracted light and higher-order diffracted lights in the pupil of the projection optical system.
Therefore, in order to enlarge the apparent focal depth for isolated patterns, e.g., contact holes, an exposure method has been proposed in which for each shot region on a wafer, the wafer is stepwisely moved along the optical axis by a predetermined amount at a time, and exposure is carried out for each stop position of the wafer, that is, exposure is carried out a plurality of times for each shot region. For example, see Japanese Patent Unexamined Publication (KOKAI) No. 63-42122 (corresponding to U.S. Pat. No. 4,869,999). This exposure method is called FLEX (Focus Latitude enhancement EXposure) method and provides satisfactory focal depth enlarging effect for isolated patterns, e.g., contact holes. However, the FLEX method indispensably requires multiple exposure of contact hole images which are slightly defocused. Therefore, the sharpness a composite optical image obtained by the multiple exposure and a resist image obtained after development inevitably decreases. Accordingly, the FLEX method suffers from problems such as degradation of the resolution of contact hole patterns which are close to each other, and lowering of the margin for the variation of the exposure degree (i.e., exposure margin).
It should be noted that the FLEX method is also disclosed in Japanese Patent Unexamined Publication (KOKAI) No. 05-13305 (corresponding to U.S. Pat. application Ser. No. 820,244, filed by the present applicant; in which the wafer is moved along the optical axis not stepwisely but continuously during exposure), and U.S. Pat. No. 5,255,050.
As another conventional technique, there has recently been proposed a technique in which a pupil filter is provided in a pupil plane of a projection optical system, that is, a plane of the projection optical system that is in Fourier transform relation to both the reticle pattern surface and the wafer surface, in an image-forming optical path between the reticle and the wafer, thereby improving the resolution and the focal depth. Examples of this technique include the Super-FLEX method published in Extended Abstracts (Spring Meeting, 1991) 29a-ZC-8, 9; The Japan Society of Applied Physics. This method is also disclosed in EP-485062A. In the Super-FLEX method, a transparent phase plate is provided at the pupil of a projection optical system so that the complex amplitude transmittance that is given to image-forming light by the phase plate successively changes from the optical axis toward the periphery in the direction perpendicular to the optical axis. By doing so, the image that is formed by the projection optical system maintains its sharpness with a predetermined width (wider than that in the conventional method) in the optical axis direction about the best focus plane (a plane that is conjugate with respect to the reticle) which is the center of said predetermined with. Thus, the focal depth increases. It should be noted that the pupil filter used in the Super-FLEX method, that is, so-called multifocus filter, is detailed in the paper entitled "Research on Imaging Performance of Optical System and Method of Improving the Same", pp.41-55, in Machine Testing Institute Report No. 40, issued on Jan. 23, 1961. For the pupil filter itself, please see U.S. Pat. No. 5,144,362.
However, the conventional Super-FLEX method suffers from the problem that the intensity of a subsidiary Peak (ringing) which occurs in the vicinity of a contact hole pattern becomes relatively strong, although the method provides satisfactory focal depth increasing effect for isolated contact hole patterns. Therefore, in the case of a plurality of contact hole patterns which are relatively close to each other, an undesirable ghost pattern is transferred to a position where ringings occurring between adjacent holes overlap each other, causing an undesired reduction in film thickness of the photoresist.